Switched mode power supplies can be used to create a direct current (DC) voltage from an alternating current (AC) voltage by switching current through an energy storage element such as a transformer. The duty cycle of the switching is controlled to regulate the output voltage to a desired level. Switched mode power supplies are generally efficient at heavier loads but less efficient at lighter loads. Two popular types of isolated switched mode power supplies are forward mode and flyback mode converters.
Flyback converters are common in AC voltage to DC voltage applications. A flyback converter is based on a flyback transformer that alternately builds up flux in the magnetic core and transfers energy to the output. When current is switched through the primary winding, the primary current in the transformer increases, storing energy within the transformer. When the switch is opened, the primary current in the transformer drops, inducing a voltage on the secondary winding. The secondary winding supplies current into the load. A controller varies the on- and off-times of a primary switch in series with the primary winding to regulate the output voltage to a desired level.
Flyback converters can be configured to switch additional reactive elements in parallel to the primary winding using a topology known as active clamp flyback (ACF). ACF converters can reduce electric stress on components and improve efficiency by achieving close to zero volt switching (ZVS) of the primary switch and to produce clean drain waveforms without any ringing. They also allow soft increase in secondary current. However while ACF converters have high efficiency at medium and heavy loads, their efficiency decreases at lighter loads due to continuous conduction losses from magnetizing current that continuously circulate on the primary side of the transformer due to the additional reactive elements. Moreover, ACF converters are not suitable for other techniques that improve efficiency at light loads such as cycle skipping and frequency foldback.
In the following description, the use of the same reference numerals in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.